For image quality, peak white is of paramount importance. The Peak White Enhancement Factor (PWEF) can be defined as the ratio between the peak white luminance, to the luminance of a homogeneous white field, usually referred to as the full white level. CRT based displays have PWEFs of up to 5, first generation of PDPs were characterised by having a peak white to maximum average luminance ratio of about 2. This is far worse than what is achieved in old CRT technology.
A Plasma Display Panel (PDP) utilizes a matrix array of discharge cells, which could only be “ON” or “OFF”. Also unlike a CRT or LCD in which grey levels are expressed by analogue control of the light emission, a PDP controls the grey level by modulating the number of light pulses per frame (sustain pulses). The eye will integrate this time-modulation over a period corresponding to the eye time response.
More sustain pulses correspond to higher peak luminance values. More sustain pulses correspond also to a higher power that flows in the PDP. The PDP control can generate more or less sustain pulses as a function of average picture power, i.e., it switches between modes with different power levels. In this document, the Power Level of a given mode is defined as the number of sustain discharges activated for a region of 100 ire video. The available range of power level modes is regarded as approximately equal to the PWEF.
A previous European patent application of the applicant with application number 99101977.9 reports a technique that increases the PWEF of a PDP by increasing the number of available power level modes, in number and in range, and by introducing an hysteresis circuit in the luminance level selection control. This technique allows achieving PWEF values up to 5.
PDPs have a large surface. A PWEF of 5, although pleasant to the image quality, has the disadvantage that it may concentrate, under certain circumstances, for a long time, the power dissipation on a small surface of the panel. If this situation is prolonged for a long time, which may occur in case of still video, local overheating of the panel may assume unacceptable values.
It has been proposed in WO 99/30309 to provide a panel temperature detector beside an average picture level detector and a picture peak level detector in a PDP for the purpose of power level control.